The invention relates to a method for manufacturing a cathode for a molten carbonate fuel cell by electrochemical oxidation of a porous metallic precursor electrode and contact with molten carbonate, and to a cathode manufactured by the method.
As a rule, porous cathodes made of lithiated nickel oxide are used in molten carbonate fuel cells. It is known from the book "Fuel Cell Handbook" by A. J. Appleby and F. R. Foulkes, Van Nostrand Reinhold, New York, page 546, to shape these cathodes in the fuel cell itself from porous nickel precursor electrodes that are in contact with the molten flux electrolytes, by oxidation with atmospheric oxygen at 600 to 650.degree. C. when putting the cells and/or cell stacks into service. As the chemical reaction EQU Ni+1/2O.sub.2 .fwdarw.NiO (1)
proceeds, lithium oxide from the melt is incorporated into the nickel oxide toward the end of this oxidation process, and this eventually lends the nickel oxide the p-semiconductor property (specific conductivity approximately 20 S.cndot.cm.sup.-1 at 650.degree. C.) required for its function.
The speed of this formation process determines the morphological properties of the cathode which in turn the nickel oxide toward the end of this oxidation process, and this eventually lends the nickel oxide the p-semiconductor property (specific conductivity approximately 20 S.cndot.cm.sup.-1 at 650.degree. C.) required for its function.
The speed of this formation process determines the morphological properties of the cathode which in turn influence the functional ability and quality of the cathode to a critical degree.
It is desirable to form relatively large porous agglomerates composed of tiny nickel oxide crystals and to form relatively wide pores between the agglomerates. If possible, the small crystals should have diameters of at least a tenth of a micron, while the average diameter of the agglomerates and coarse pores should optimally be between 1 and 7 microns.
A relatively high reaction rate of oxidation favors the formation of the desired cathode structure and morphology, while oxidation of the metal that is too slow, and which can extend for example over periods of more than ten hours, leads to the formation of very coarse nickel oxide crystals which can then measure several microns in diameter, and to the failure of the desired agglomerates to form. The oxidation rate of the nickel metal can be influenced by the oxygen content and the flowrate of the forming gas.
Precise control of the oxidation rate of the nickel metal is difficult for two reasons.
(1) Frequently, heat stagnation caused by the release of the reaction enthalpy of the exothermic reaction explained above causes the formation of temperature peaks in the surface of the cell. To avoid this overheating, the oxygen content of the forming gas must be reduced to a point at which undesirably long reaction times of more than ten hours are no longer avoidable.
(2) It is unavoidable that as a result of leaks in the cell that are statistically distributed over the surface, a small amount of anode gas which has a reducing action crosses over to the cathode side at locally closely limited points and delays the oxidation of the nickel metal there in the vicinity of the point at which it penetrates, so that locally limited formation of undesired coarse nickel oxide structures occurs at that location because of the retarded oxidation of the nickel, and very low current densities are formed during cell operation, in other words there is an undesired nonuniform current density distribution that adversely affects the overall operation of the cell stack.